This invention relates generally to the fabrication of integrated circuits on semiconductor wafers, and more particularly to the logistics of moving wafers for processing during the fabrication.
A fabrication plant is usually divided into separate areas for particular portions of the fabrication process. Each processing area is called a bay, and each bay contains equipment that is required to process wafers for a certain purpose. In addition to the equipment, material stockers are present near the bay. A material stocker is a large automated cabinet where wafers are stored while waiting to be processed. The wafers are stored in cassettes, and each cassette can hold up to twenty-five wafers. A lot is a logical grouping of wafers in a cassette. The lot can migrate from cassette to cassette as it progresses through the chain of production processes.
A material stocker can hold hundreds of cassettes and services two bays. When a cassette of wafers is ready to be stored in a material stocker, the operator places the cassette at one of the input ports of the material stocker.
When an operator wishes to retrieve a lot from a material stocker, he issues a request via the application program of the host computer. The operator may need empty cassettes to configure or process the wafers. Empty cassettes may be stored in a material stocker, so the operator must have the capability to request empty cassettes. In addition to manipulation of the wafers, the operator may also need to manipulate, or set up, the equipment for an operation.
Once a lot has been retrieved, and the equipment has been set up, the operation on the wafers by a particular piece of equipment, or "tool," can begin. At this point, the lot is moved-in to the operation. This state is indicated to the host application by the operator for the lot. The lot remains in this state until the operation is completed. Once the operation is completed, the operator must perform tests and verifications on the wafers. When all tests and verifications have been performed, the host computer application program must be notified. Wafers may have moved from one cassette to another as a result of the operation. The host application has to be notified of this. The operator then places the cassette of "moved-out" wafers in the material stocker, to await orders as to the location of the next piece of equipment which will perform operations on the wafers.
Some present material handling systems exhibit excess movement of materials through the system which negatively impacts manufacturing throughput. In an example system, there are multiple locations that are available for a particular manufacturing process step. However, the system only provides mappings for one location per manufacturing process step. Therefore, the system does not know that there are alternate locations available for performing the processing step. When the AMHS dispatches material to a particular location, and the stocker associated with that location is unavailable, the material is redirected to an adjacent stocker or temporary storage. If other locations are available to perform the processing step, a move request is required to transfer the material from a stocker, such as the adjacent stocker, to the desired location. The AMHS then moves the material to the stocker at the specified location, and the material is then removed and processed. Therefore, in order to move the material to the location where it is ultimately processed in the example scenario, two moves were required. A first move was required to move the material to the location adjacent to the programmed location, and the second move was required to transfer the material to the location where the material was ultimately processed. It would be desirable, therefore, to reduce the movement of material in an AMHS.